In a client-server system, a server can host an application that provides a service to a client, and the client can operate the application remotely. For example, a website, such as yahoo.com or msn.com can run applications that provide a financial portfolio tracking service to remote clients. The clients can connect to the hosted system through a network and operate the system, enter information concerning their portfolios and then track the performance of their portfolios through use of the hosted system. Because the application is hosted and run by the server and data about the users' portfolios is stored on the server, the client need not install any software or store any data about the portfolios. Moreover, the application can be updated automatically by the server, without the client having to take action to install updated or additional software. From the perspective of the website owner, an advantage of providing the hosted application in an application service provider (ASP) model, as opposed to providing the application through a stand-alone application that is loaded onto and run by the client, is that the website owner can more easily update the application, communicate with the client, and has access to data provided by the client. These advantages allow the website owner to know and understand the user better, so that the application can be tuned for the user. In addition, because the user must connect to the server to access and use the application, the website owner has the opportunity to communicate with the user more frequently.
Some hosted applications provided by a server to a client can include an electronic mail (“e-mail”) application that allows the user of the application to send e-mail messages through the hosted application. E-mail messages are composed in the e-mail application and sent from a mail server that is associated with the server. For example, if the hosted application is provided by a server associated with the domain SAP.com, the e-mail messages are sent from a mail server that is also associated with the domain SAP.com. A problem with this is that if a user utilizes the e-mail application of the hosted application to send malicious or spam e-mails, network security systems and software may associate the malicious or spam e-mail with the domain from which they were sent (i.e., SAP.com) and may take action to quarantine or blacklist the domain. Therefore, one malicious user or spam sender utilizing the hosted application may cause the e-mail traffic from non-malicious users of the hosted application or other users associated with the domain to be limited or blocked. Furthermore, a SPAM filter may identify an e-mail as SPAM if the domain of the listed as the sender's domain does not match the domain of the sending mail server. Thus, legitimate e-mails may be routed into junk mail when using a hosted solution, if the user sends e-mail from the domain of hosted solution's mail server but the user's “from” address has a different domain. In addition, if the domain of the hosted application is used to send a sufficiently large number of e-mails on behalf of its clients, then according to some laws (e.g., German Law), the provider of this hosted application may be deemed an Internet Service Provider, which may require that the that the provider conform to certain burdensome and/or costly regulations.